Hybrid vehicles including an engine and a motor as driving sources and a secondary battery as an electric power supply, and electrically-powered vehicles such as electric vehicles are capable of EV traveling, i.e. traveling in which the motor is driven only with electric power stored in the secondary battery and without using the engine for travelling. As the EV travelling is quiet and pollution free, extension of a distance traveled with the EV travelling is desired.
In some types of hybrid vehicles, it is possible to charge the secondary battery externally from outside the vehicle. In these types of hybrid vehicles, while it is necessary to increase the capacity of a secondary battery which is mounted in order to extend the distance of EV travelling, the volume of secondary batteries loaded should be limited in consideration of the power performance and interior comfort of a vehicle. Consequently, as described in JP 8-154307 A, for example, the EV travelling distance depends on the remaining capacity of the secondary battery, and can be extended if the second battery is charged up to a full-charge capacity at the time of charge.
Further, in lithium ion batteries, when a voltage between positive and negative electrode terminals is higher than the rated voltage, degradation occurs, in which a full-charge capacity (reversible capacitance) is lowered. JP 10-142302 A describes a method of calculating such a degradation amount to obtain the remaining capacity.
Here, in lithium ion batteries, there are cases where the battery is degraded when a high charge capacity state continues. The degradation of the battery results in a problem that the full-charge capacity is lowered to thereby shorten the EV travelling distance.
JP 8-154307 A describes an invention which aims at reducing travelling which depends on an internal combustion engine in a hybrid vehicle to thereby encourage more active use of EV travelling, and includes no description concerning degradation of the secondary battery. Further, JP 10-142302 A describes an invention which aims at estimating the degradation amount of a lithium ion battery to thereby accurately detect the remaining capacity, and includes no description concerning a solution to degradation of the secondary battery itself.
It is therefore an advantage of the present invention to suppress degradation of a secondary battery due to charging. Another advantage of the present invention is to extend the EV travelling distance as well as to suppress degradation of a secondary battery due to charging.